Interstellar ice analogs: band strengths of H₂O, CO₂, CH₃OH, and NH₃in the far-infrared region

Abstract

Context. Whereas observational astronomy now routinely extends to the far-infrared region of the spectrum, systematic laboratory studies are sparse. In particular, experiments on laboratory analogs performed through the years have provided information mainly about the band positions and shapes, while information about the band strengths are scarce and derivable principally from the optical constants. Aims. We measure the band strengths in the far-infrared region of interstellar ice analogs of astrophysically relevant species, such as H 2 O, CO 2 , CH 3 OH, and NH 3 , deposited at low temperature (8-10 K), followed by warm-up, to induce amorphous-crystalline phase transitions when relevant. Methods. The spectra of pure H 2 O, NH 3 , and CH 3 OH ices have been measured in the near-, mid-and far-infrared spectroscopic regions using the Interstellar Astrochemistry Chamber (ISAC) ultra-high-vacuum setup. In addition, far-infrared spectra of NH 3 and CO 2 were measured using a different set-up equipped with a bolometer detector. Band strengths in the far-infrared region were estimated using the corresponding near-and mid-infrared values as a reference. We also performed theoretical calculations of the amorphous and crystalline structures of these molecules using solid state computational programs at density functional theory (DFT) level. Vibrational assignment and mode intensities for these ices were predicted. Results. Infrared band strengths in the 25-500 μm range have been determined for the considered ice samples by direct comparison in the near-and mid-infrared regions. Our values were compared to those we calculated from the literature complex index of refraction. We found differences of a factor of two between the two sets of values. Conclusions. The calculated far-infrared band strengths provide a benchmark for interpreting the observational data from future space telescope missions, allowing the estimation of the ice column densities.

Interstellar ice analogs: H2O ice mixtures with CH3OH and NH3in the far-IR region

“…The calculated band at 444 cm −1 is assigned to a librational mode from both ammonia and water. Although both species contribute to both vibrational modes, the contributions are not equivalent; there is a larger participation of the water motion in the lower frequency mode and of ammonia in the second, which is in agreement with the assignment proposed for the spectra of the pure ices (Giuliano et al 2014).…”

“…(c) Band strength of the related band in the pure ice, from Giuliano et al (2014). (d) The band corresponding to amorphous water is blended with the wide undetected band of methanol at ∼303 cm −1 .…”

“…(a) Peak frequency of the related band in the pure ice, from Giuliano et al (2014). (b) Derived from the analysis of the bands in the far-IR region.…”

“…However, band strengths are still needed for quantification purposes. In Giuliano et al (2014) the infrared band strengths in the 25−500 µm region were measured for pure ices of the most astrophysically relevant species, namely H 2 O, CO 2 , CH 3 OH, and NH 3 , at a temperature of 8 K followed by warm up. Despite their presence in interstellar ices, CO and CH 4 do not present any feature in the far-IR region above 100 cm −1 and were not included in the study.…”

“…We did not study binary mixtures with carbon dioxide since the CO 2 far-IR features fall in a frequency region where the noise level is too high (see Giuliano et al 2014). The presence of another component in a pure ice is known to affect the position and shape of the vibrational bands and, to some extent, can alter their band strengths.…”